The present invention relates to a heat-resistant inorganic pigment and a process for producing the same, and more particularly, it relates to a heat-resistant inorganic pigment assuming various hues and a process for producing the same. The pigment of the present invention is useful as a pigment for a heat-resistant coating material and is a novel heat-resistant inorganic pigment which does not pollute the environment.
As a representative of composite metal oxides containing one divalent metal M and Ti, ilmenite (titanic iron) will be cited in which the divalent metal M is Fe. Composite metal oxides which contain one selected from the group consisting of Mn, Mg, Ni and Co as the divalent metal M and Ti are also reported in J. Phys. Soc. Jpn. Vol. 11, No. 5, published in May, 1956 and ibiden Vol. 13, No. 10, published in October 1958 as a composite metal oxide which contains other than Fe as the divalent metal M.
Any of these composite metal oxides such as ilmenite, etc., which contain one divalent metal M and Ti is obtained by heating and calcining at a temperature of not lower than 1350.degree. C., as described in the above literature.
Japanese Patent Application Laid-Open (KOKAI) Nos. 1-298028 (1989), 4-325419 (1992) and 4-55324 (1992) disclose methods of synthesizing FeTiO.sub.3 particles, MnTiO.sub.3 particles and CoTiO.sub.3 particles, respectively, in an autoclave.
The ilmenite particles disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 1-298028 (1989) are individual FeTiO.sub.3 particles which have an average particle diameter of 0.05 to 2.0 .mu.m and which have a uniform particle size; the pigment particles disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 4-325419 (1992) are individual MnTiO.sub.3 particles which have an average particle diameter of 0.5 to 2.0 .mu.m and which have a uniform particle size; and the pigment particles disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 4-55324 (1992) are individual CoTiO.sub.3 particles which have an average particle diameter of 0.5 to 2.0 .mu.m and which have a uniform particle size.
It is widely known that each composite metal oxide MTiO.sub.3 (M represents a divalent metal) represented by ilmenite, MnTiO.sub.3 or COTiO.sub.3 has a good heat resistance, but each has a monotonous hue and lacks in variety. For example, the hues of the composite metal oxides of FeTiO.sub.3, MnTiO.sub.3 or CoTiO.sub.3 particles obtained by the methods of synthesis described in Japanese Patent Application Laid-Open (KOKAI) Nos. 1-298028 (1989), 4-325419 (1992) and 4-55324 (1992), are black, yellow and green.
Although it is described in the afore-mentioned Japanese KOKAIs that the heat resistance of each composite metal oxide is not lower than 150.degree. C., since it is obtained by synthesizing at a temperature of not higher than 300.degree. C. as described therein, the upper limit of the heat resistance is about 300.degree. C. (refer to Comparative Example described later). These composite metal oxides are, therefore, used in the field of general coating materials and toners, and use thereof in the field which requires a heat resistance exceeding 300.degree. C. is not disclosed nor taught.
Accordingly, a technical problem of the present invention is to provide at the production of a novel heat-resistant inorganic pigment which assumes various sharpness hues and is rich in variety, which is usable in the field requiring a heat resistance of not lower than 300.degree. C., and which does not pollute the environment.
As a result of earnest studies undertaken by the present inventor to solve the afore-mentioned problem, it has been found that by mixing compounds of two divalent metals selected from the group consisting of Mg, Fe, Ni and Co with TiO.sub.2, heating and calcining the obtained mixture at a temperature of not more than 1350.degree. C., and pulverizing the calcined material, the obtained particles containing a specified ratio of two divalent metals selected from the group consisting of Mg, Fe, Ni and Co, and Ti, assume various sharpness hues and are usable as a pigment in the field which requires a heat resistance of not lower than 300.degree. C. On the basis of this finding, the present invention has been achieved.